This invention relates to transmission controls and more particularly to transmission controls for automatically shifted transmissions.
In automatically shifted transmissions, it is well-known to provide selectively operable friction devices which will establish various drive ratios between the transmission input and the transmission output. It is not desirable to have these friction devices continuously and simultaneously engaged, if during engagement, an attempt to establish more than one drive ratio occurs.
In many planetary type transmissions, this is prevented by utilizing a band brake to establish one transmission ratio and a disc clutch to establish the other. In these arrangements, it is common to utilize the on-going clutch pressure to disengage the band brake. In other hydraulic schemes, it is common to utilize a shift valve which will direct fluid to one or the other of two friction devices, but not both simultaneously, thereby preventing the simultaneous establishment of these devices.
With the advent of electronically controlled transmissions, wherein each friction device is controlled by a separate solenoid valve, it becomes exceedingly difficult to ensure that the simultaneous establishment of more than one drive ratio will not occur.